Conventional screen-filters are used in many gas, oil and water wells to reduce sanding and silting in. These screen filters generally employ as filter material any of an artificial gravel pack poured into an annular space between the wall of the hole and the outside space of the screen, or a gravel pack formed by the formation gravel grains during development of the wells or gravel grains or grains formed of other materials, so that the filter material is applied as a bonded-on pack into the slots of the screen or bonded-on as a coated pack on the outside space of a single screen with a special type of resin.
The wells with a gravel pack in the annular space must be drilled with a large diameter to accommodate a minimum of three inch gravel pack thickness from each side and to ensure that a required envelope of gravel will surround the entire screen. But generally, even in the large diameter hole having an adequate annular space, gravel is improperly packed. Therefore, this approach is costly and not effective to prevent sanding and silting in, particularly for deeper wells.
The pack formed by development from formation gravel grains, also requires a large hole diameter and a very strong and special type of screen for long lasting development and very often fails to be fully satisfactory. Therefore, this approach is more costly and is not effective for protection of sanding and silting in.
A pack bonded-on the outside or bonded-on into the screen openings from the gravel grains or grains from the other materials is very costly, fragile and very sensitive to transportation and installation into the hole.
Such tightly packed grains reduce by about three times the free flow areas, compared with that of a loosely packed grains.
Fine sand and silt entering any of these gravel or other packs will drastically reduce porosity and permeability of the filtering system. When fine particles occupy openings between the coarser particles of the artificial gravel pack, i.e., when porosity and permeability are reduced, the velocity of the fluid flowing through the screen-filter will be drastically increased, causing abrasion and incrustation of the screen, reduction of the yield, with increased possibility to the collapse of the screen or casing.
Cleaning and maintenance of such well screen-filters are very costly and hazardous. Therefore, the key to good well efficiency is effective screen-filter operation which depends in turn upon the screen design.